arXiv subjects

Zhiyu Zhang

Publications and source records attributed to Zhiyu Zhang.

88 records · Page 5Linked to original sources

arXiv2017

Bias-Compensated Normalized Maximum Correntropy Criterion Algorithm for System Identification with Noisy Input

This paper proposed a bias-compensated normalized maximum correntropy criterion (BCNMCC) algorithm charactered by its low steady-state misalignment for system identification with noisy input in an impulsive output noise environment. The normalized maximum correntropy criterion (NMCC) is derived from a correntropy based cost function, which is rather robust with respect to impulsive noises. To deal with the noisy input, we introduce a bias-compensated vector (BCV) to the NMCC algorithm, and then an unbiasedness criterion and some reasonable assumptions are used to compute the BCV. Taking advantage of the BCV, the bias caused by the input noise can be effectively suppressed. System identification simulation results demonstrate that the proposed BCNMCC algorithm can outperform other related algorithms with noisy input especially in an impulsive output noise environment.

stat.ML↗

arXiv2017

Simultaneous summation and recognition effects for a dual-emitter light-induced neuromorphic device

We propose and fabricate a dual-emitter light-induced neuromorphic device composed of two light-induced devices with a common collector and base. Two InGaN multiple quantum well diodes (MQW-diodes) are used as the emitters to generate light, and one InGaN MQW-diode is used as the common collector to absorb the emitted light. When the presynaptic voltages are synchronously applied to the two emitters, the collector demonstrates an adding together of the excitatory post synaptic voltage (EPSV). The width and period of the two input signals constitute the code to generate spatial summation and recognition effects at the same time. Experimental results confirm that temporal summation caused by the repetitive-pulse facilitation could significantly strengthen the spatial summation effect due to the adding together behavior when the repetitive stimulations are applied to the two emitters in rapid succession. Particularly, the resonant summation effect occurs at the co-summation region when the two repetitive-pulse signals have a resonant period, which offers a more sophisticated spatiotemporal EPSV summation function for the dual-emitter neuromorphic device.

physics.app-ph↗

arXiv2016

Cloud Structure of Galactic OB Cluster Forming Regions from Combining Ground and Space Based Bolometric Observations

We have developed an iterative procedure to systematically combine the millimeter and submillimeter images of OB cluster-forming molecular clouds, which were taken by ground based (CSO, JCMT, APEX, IRAM-30m) and space telescopes (Herschel, Planck). For the seven luminous ($L$$>$10$^{6}$ $L_{\odot}$) Galactic OB cluster-forming molecular clouds selected for our analyses, namely W49A, W43-Main, W43-South, W33, G10.6-0.4, G10.2-0.3, G10.3-0.1, we have performed single-component, modified black-body fits to each pixel of the combined (sub)millimeter images, and the Herschel PACS and SPIRE images at shorter wavelengths. The $\sim$10$"$ resolution dust column density and temperature maps of these sources revealed dramatically different morphologies, indicating very different modes of OB cluster-formation, or parent molecular cloud structures in different evolutionary stages. The molecular clouds W49A, W33, and G10.6-0.4 show centrally concentrated massive molecular clumps that are connected with approximately radially orientated molecular gas filaments. The W43-Main and W43-South molecular cloud complexes, which are located at the intersection of the Galactic near 3-kpc (or Scutum) arm and the Galactic bar, show a widely scattered distribution of dense molecular clumps/cores over the observed $\sim$10 pc spatial scale. The relatively evolved sources G10.2-0.3 and G10.3-0.1 appear to be affected by stellar feedback, and show a complicated cloud morphology embedded with abundant dense molecular clumps/cores. We find that with the high angular resolution we achieved, our visual classification of cloud morphology can be linked to the systematically derived statistical quantities (i.e., the enclosed mass profile, the column density probability distribution function, the two-point correlation function of column density, and the probability distribution function of clump/core separations).

astro-ph.GA↗

arXiv2011

The Origin of OB Clusters: From 10 pc to 0.1 pc

We observe the 1.2 mm continuum emission around the OB cluster forming region G10.6-0.4, using the IRAM 30m telescope MAMBO-2 bolometer array and the Submillimeter array. Comparison of the Spitzer 24 $μ$m and 8 $μ$m images with our 1.2 mm continuum maps reveals the ionization front of an HII region, the photon-dominated layer, and several 5 pc scale filaments following the outer edge of the photon-dominated layer. The filaments, which are resolved in the MAMBO-2 observations, show regularly spaced parsec-scale molecular clumps, embedded with a cluster of submillimeter molecular cores as shown in the SMA 0.87 mm observations. Toward the center of the G10.6-0.4 region, the combined SMA+IRAM 30m continuum image reveals several, parsec-scale protrusions. They may continue down to within 0.1 pc of the geometric center of a dense 3 pc size structure, where a 200 M$_{\odot}$ OB cluster resides. The observed filaments may facilitate mass accretion onto the central cluster--forming region in the presence of strong radiative and mechanical stellar feedbacks. Their filamentary geometry may also facilitate fragmentation. We did not detect any significant polarized emission at 0.87 mm in the inner 1 pc region with the SMA.

astro-ph.SR↗

arXiv2011

CS 5-4 survey toward nearby IR bright galaxies

With the observations of the CS 5-4 line toward a sample of 24 infrared bright galaxies using HHSMT, we detected CS 5-4 emission in 14 galaxies, including 12 ULIRGs/LIRGs and 2 nearby normal galaxies. As a good dense gas tracer, which has been well used for studying star formation in the Milky Way, CS 5-4 can trace the active star forming gas in galaxies. The correlation between CS 5-4 luminosity, which is estimated with detected CS 5-4 line emission, and infrared luminosity in these 14 galaxies is fitted with correlation coefficient of 0.939 and the slope close to unity. This correlation confirmed that dense gas, which is closely linked to star formation, is very important for understanding star formation in galaxies.

astro-ph.CO↗

arXiv2011

12CO, 13CO and C18O observations along the major axes of nearby bright infrared galaxies

We present simultaneous observations of CO,13CO and C18O J=1-0 emission in 11 nearby (cz<1000 km/s) bright infrared galaxies. Both 12CO and 13CO are detected in the centers of all galaxies, except for 13CO in NGC 3031. We have also detected C18O, CS J=2-1, and HCO+ J=1-0 emission in the nuclear regions of M82 and M51. These are the first systematical extragalactic detections of 12CO and its isotopes from the PMO 14m telescope. We have conducted half-beam spacing mapping of M82 over an area of 4'*2.5' and major axis mapping of NGC 3627, NGC 3628, NGC 4631, and M51. The radial distributions of 12CO and 13CO in NGC 3627, NGC 3628, and M51 can be well fitted by an exponential profile. The 12CO/13CO intensity ratio,R,decreases monotonically with galactocentric radius in all mapped sources. The average R in the center and disk of the galaxies are 9.9+/-3.0 and 5.6+/-1.9 respectively, much lower than the peculiar R(~24) found in the center of M82. The intensity ratios of 13CO/C18O, 13CO/HCO+ and 13CO/CS (either ours or literature data) show little variations with galactocentric radius, in sharp contrast with the greatly varied R. This supports the notion that the observed gradient in R could be the results of the variations of the physical conditions across the disks. The H_2 column density derived from C18O shows that the Galactic standard conversion factor (X-factor) overestimates the amount of the molecular gas in M82 by a factor of ~2.5. These observations suggest that the X-factor in active star-forming regions (i.e., nuclear regions) should be lower than that in normal star-forming disks, and the gradient in R can be used to trace the variations of the X-factor.

astro-ph.CO↗

arXiv2010

Heat treatment effects on the superconducting properties of Ag-doped SrKFeAs compound

The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were strongly influenced by Ag doping (Supercond. Sci. Technol. 23 (2010) 025027). Ag addition is mainly dominated by silver diffusing, so the annealing process is one of the essential factors to achieve high quality Ag doped Sr0.6K0.4Fe2As2. In this paper, the optimal annealing conditions were studied for Ag doped Sr0.6K0.4Fe2As2 bulks prepared by a one-step solid reaction method. It is found that the annealing temperature has a strong influence on the superconducting properties, especially on the critical current density Jc. As a result, higher heat treatment temperature (~900C) is helpful in diffusing Ag and reducing the impurity phase gathered together to improve the grain connectivity. In contrast, low-temperature sintering is counterproductive for Ag doped samples. These results clearly suggest that annealing at ~900C is necessary for obtaining high Jc Ag-doped samples.

cond-mat.supr-con↗

arXiv2010

Low-temperature synthesis of SmO0.8F0.2FeAs superconductor with Tc = 56.1K

We report a systematic study on the effect of sintering temperature on the structural and superconducting properties of nominal SmO0.8F0.2FeAs fabricated by simple one-step solid state reaction method. A detailed correlation between the sintering temperature, structure, onset transition temperature (Tc) and critical current density (Jc) has been found in all samples of each batch. Most importantly, samples sintered at a low temperature clearly shows high Tc, for example the Tc of the samples sintered at 850C is even above 53 K, and the samples prepared at 1000C display the highest Tc of 56.1 K reported so far. Furthermore, the samples sintered at 1000C also show the highest RRR and the lowest resistivity(57K), indicating the low impurity scattering and enhanced carrier density. However, the maximum Jc of 10510 A/cm^2 at 5 K in self field was achieved in the samples sintered at 1100oC. The dependence of Tc on a-axis lattice constant indicates that the sintering temperature has strong influence on the effective substitution level of F for O. This result suggests that annealing at a temperature of ~1000C seems much better for obtaining high quality 1111 phase oxypnictides, compared to commonly used temperatures of around 1200C.

cond-mat.supr-con↗

arXiv2010

Low-temperature synthesis of SmFeAsO0.7F0.3 wires with high transport critical current density

Ag-sheathed SmFeAsO0.7F0.3 (Sm-1111) superconducting wires were prepared by a one-step solid state reaction at temperatures as low as 850~900C, instead of commonly used temperatures of 1150~1250C. The X-ray diffraction pattern of the as-sintered samples is well indexed on the basis of tetragonal ZrCuSiAs-type structure. We characterized transport critical current density Jc of the SmFeAsO0.7F0.3 wires in increasing and subsequently decreasing fields, by a resistive four-probe method. A transport Jc as high as ~1300 A/cm^2 at 4.2 K and self field has been observed for the first time in Sm-1111 type polycrystalline superconductors. The Jc also shows a rapid depression in small applied fields as well as a magnetic-history dependence, indicating weak-linked grain boundaries. The low-temperature synthesis method can be very beneficial to fabricating the RE-1111 iron oxynictides in a convenient and safe way.

cond-mat.supr-con↗

arXiv2010

Influence of Pb addition on the superconducting properties of polycrystalline Sr0.6K0.4Fe2As2

Polycrystalline Sr0.6K0.4Fe2As2 samples with various Pb additions (0-20 wt%) were prepared using a one-step solid state reaction. X-ray diffraction analysis shows no evidence for chemical reaction between the Pb and the FeAs-based superconductor. However, the presence of the Pb can affect the microstructure and superconducting properties of the final products. The critical transition temperature Tc indicates no degradation up to 20 wt% Pb addition, and dramatic improvements of magnetic Jc and irreversibility field Hirr were observed for appropriate Pb concentration. Transport critical current property of pure and Pb-added Sr0.6K0.4Fe2As2 tapes was also measured by a four-probe technique, and a remarkable enhancement of Jc at low fields was detected for the Pb added tapes.

cond-mat.supr-con↗

arXiv2009

CO observations of SNR IC 443

We present our 12CO and 13CO mapping observations of SNR IC 443, interacting with molecular clouds. It is the first large scale high resolution 13CO mapping observation in the surrounding region. The morphologies of IC 443 in 12CO and 13CO are compared with the optical, infrared, Spitzer far-infrared, X-ray, and neutral atomic gas (HI). We also make comparison and analysis in the kinemics, using the date-cubes of 12CO, 13CO and HI, to help distinguish the complicated gas motions in the shocked regions. Based on the work of Wang & Scoville (1992), we present a new model to explain the origin of coexistence of shocks with different speeds in a rather small region at the central clump B. We test this new model via analyzing the HI and CO distributions in both velocity and space domains. We also establish the relationship between the dissociation rate of the shocked molecular gas and the shock velocity in this region. Finally, we derive the optical depth of 12CO with the 13CO spectra in clump B, and discuss the validity of the assumption of optically thin emission for the shocked 12CO.

astro-ph.GA↗

arXiv2009

Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

We report significant transport critical currents firstly achieved in Sr0.6K0.4Fe2As2 wires and tapes with a Tc = 34 K, which were fabricated through an in-situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. Transport measurements were performed by a standard four-probe resistive method. All the wire and tape samples have shown transport properties. Critical current density Jc was enhanced upon silver addition, and at 4.2 K, a best Jc of ~1200 A/cm^2 (Ic = 9 A) was achieved for 20 % silver added tapes, which is the highest in iron-based wires and tapes so far. The Jc is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the absence of reaction layer between the silver sheath and superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the creep drop of Jc at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.

cond-mat.supr-con↗

arXiv2009

Superconductivity in Ir-doped LaFe1-xIrxAsO

We report the realization of superconductivity by 5d element Ir doping in LaFeAsO, a prototype parent compound of high-temperature iron based superconductors. X-ray diffraction patterns indicate that the material has formed the ZrCuSiAs-type structure with a space group P4/nmm. The systematic evolution of the lattice constants demonstrated that the Fe ions were successfully replaced by the Ir. Both electrical resistance and magnetization measurements show superconductivity up to 11 K in LaFe1-xIrxAsO. The superconducting transitions at different magnetic fields were also measured yielding a slope of -dHc2/dT = 6.7 T / K near Tc, and the upper critical field at zero K is found to be about 55 T. This is the first example of bulk superconductivity induced by replacing the Fe sites with higher d-orbital electrons in FeAs-1111 family, which should add more ingredients to the underlying physics of the iron-based superconductors.

cond-mat.supr-con↗

arXiv2009

Fabrication and characterization of iron pnictide wires and bulk materials through the powder-in-tube method

The recent discovery of superconductivity in the iron based superconductors with very high upper critical fields presents a new possibility for practical applications, but fabricating fine-wire is a challenge because of mechanically hard and brittle powders and the toxicity and volatility of arsenic. In this paper, we report the synthesis and the physical characterization of iron pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new class of high-Tc iron pnictide composite wires, such as LaFeAsO1-xFx, SmFeAsO1-xFx and Sr1-xKxFeAs, has been fabricated by the in situ PIT technique using Fe, Ta and Nb tubes. Microscopy and x-ray analysis show that the superconducting core is continuous, and retains phase composition after wire drawing and heat treatment. Furthermore, the wires exhibit a very weak Jc-field dependence behavior even at high temperatures. The upper critical field Hc2(0) value can exceed 100 T, surpassing those of MgB2 and all the low temperature superconductors and indicating a strong potential for applications requiring very high field. These results demonstrate the feasibility of producing superconducting pnictide composite wire. We also applied the one step PIT method to synthesize the iron-based bulks, due to its convenience and safety. In fact, by using this technique, we have successfully discovered superconductivity at 35 K and 15 K in Eu0.7Na0.3Fe2As2 and SmCoFeAsO compounds, respectively. These clearly suggest that the one-step PIT technique is unique and versatile and hence can be tailored easily for other rare earth derivatives of novel iron-based superconductors.

cond-mat.supr-con↗

arXiv2009

Superconductivity in SmFe1-xMxAsO (M = Co, Rh, Ir)

In this paper we report the comparative study of superconductivity by 3d (Co), 4d (Rh), 5d (Ir) element doping in SmFeAsO. X-ray diffraction patterns indicate that the material has formed the ZrCuSiAs-type structure with a space group P4/nmm. It is found that the antiferromagnetic spin-density-wave (SDW) order in the parent compounds is rapidly suppressed by Co, Rh, and Ir doping, and superconductivity emerges. Both electrical resistance and magnetization measurements show superconductivity up to around 10 K in SmFe1-xMxAsO (M = Co, Rh, Ir). Co, Rh and Ir locate in the same column in the periodic table of elements but have different electronic band structure, so comparative study would add more ingredients to the underlying physics of the iron-based superconductors.

cond-mat.supr-con↗

arXiv2009

Superconductivity of powder-in-tube Sr0.6K0.4Fe2As2 wires

Nb-sheathed Sr0.6K0.4Fe2As2 superconducting wires have been fabricated using the powder-in-tube (PIT) method for the first time and the superconducting properties of the wires have been investigated. The transition temperature (Tc) of the Sr0.6K0.4Fe2As2 wires is confirmed to be as high as 35.3 K. Most importantly, Sr0.6K0.4Fe2As2 wires exhibit a very weak Jc-field dependence behavior even the temperature is very close to Tc. The upper critical field Hc2(0) value can exceed 140 T, surpassing those of MgB2 and all the low temperature superconductors. Such high Hc2 and superior Jc-field performance make the 122 phase SrKFeAs wire conductors a powerful competitor potentially useful in very high field applications.

cond-mat.supr-con↗